Recently, various high-functional resins or films in which zirconium oxide is combined with a transparent resin or film to improve its refractive index by making use of the high refractive index of the zirconium oxide have been proposed.
For example, it is known that when zirconium oxide having a high refractive index is added to an LED sealing resin, the refractive index of the sealing resin can be heightened, and hence it is possible to further efficiently take out emission of light from illuminant, thus resulting in improved brightness of an LED.
Similarly, zirconium oxide is also used in an antireflection film on a display surface of a flat panel display (FPD) such as a liquid crystal display (LCD), a plasma display (PDP), or an electroluminescence display (EL). The antireflection film is a laminated film in which a layer having a low refractive index and a layer having a high refractive index are laminated, and a composite resin in which zirconium oxide is dispersed is used in the layer having a high refractive index.
In the uses described above, in the case a primary particle size of zirconium oxide and a secondary particle size of aggregated particles of zirconium oxide in a resin are not sufficiently smaller than a wave length of visible rays (380 to 800 nm), a sealing resin or an antireflection film becomes clouded due to an influence of scattering caused by the particles of zirconium oxide, and thus necessary transparency cannot be obtained. It is therefore strongly required to develop a highly transparent dispersion of zirconium oxide in which the particles of zirconium oxide are dispersed as fine particles in a resin.
In order to meet the requirement, various fine particles of zirconium oxide and processes for producing a dispersion of such fine particles have been recently proposed. A typical process for obtaining a dispersion of zirconium oxide is such that it makes use of zirconium hydroxide produced by neutralizing a zirconium salt with an alkali. For example, a process is known in which hydrochloric acid having a predetermined concentration is added to a slurry of zirconium hydroxide, and the resulting mixture is heated at a boiling temperature to obtain a dispersion of zirconium oxide (Patent Literature 1). According to this process, however, the obtained zirconium oxide has an average particle size of 50 nm or more, and therefore it is difficult that the dispersion has a satisfactory transparency.
A process is also known in which an aqueous solution of a zirconium salt is added to an aqueous solution of an alkali metal hydroxide which has been heated to 60° C. or more to neutralize the alkali metal hydroxide, that is, reverse neutralization is carried out; then the resulting product is filtered, and washed; and water is added to the product; the resulting mixture is stirred; an acid is added to the mixture; and the resulting mixture is stirred while it is heated at a temperature of 80 to 100° C. to obtain a dispersion of zirconia (Patent Literature 2). According to this process, however, a long heating time is necessary, and therefore it is difficult to adopt the process for industrial production of dispersion of zirconia.
A further process is known in which an aqueous solution of alkali is added to an aqueous solution of a zirconium salt in the presence of a carboxylic acid such as malic acid, citric acid, or tartaric acid to obtain a gel of zirconium hydroxide; the gel is washed, and sufficiently dispersed by ultrasonic wave irradiation, or the like; and the resulting dispersion is hydrothermally treated in the presence of the carboxylic acid mentioned above to obtain a dispersion of zirconium oxide (Patent Literature 3). According to this process, however, the concentration of the aqueous solution of zirconium salt is limited up to 5% by weight in terms of zirconia so that the obtained zirconia sol does not have an uneven particle size, and therefore, the obtained dispersion of zirconia has a remarkably low concentration. Furthermore, because it is essential to sufficiently perform the dispersion treatment by the ultrasonic wave irradiation or the like before the hydrothermal treatment in the process, it is difficult to adopt the process for an industrial process for production of dispersion of zirconia.    Patent Literature 1: JP 1993-24844 A    Patent Literature 2: JP 2008-31023 A    Patent Literature 3: JP 2006-143535 A